I am using the Sparkfun Mp3 Player shield and a set of speakers. In addition to this shield, I am using the Shifty VU Shield to read the audio levels so that I can synchronize some LEDs with the audio decibel levels. Seperately, the two shields work perfectly. The Mp3 player shield, connected to the Arduino ONLY, can play the music through the speakers I've connected. The Shifty VU shield, connected to the Arduino ONLY, reads the audio levels sent by my phone via AUX cable while playing pandora.

The problem is that whenever I stack the two, the music simply stops working. I can't, for the life of me, figure out why it would do that. Everything is connected EXACTLY as they would be seperately, just stacked on top of eachother. Without even connecting the two through AUX, just stacking them, and the music stops playing.